The Pit of Tartarus, that was located in the Underworld, was the very site upon which the Titanomachy between the Gods and the Titans took place. After the Titans' defeat, they were imprisoned within the Pit of Tartarus for all eternity.
In the God of War Series
In God of War: Chains of Olympus, the Titans where inprisoned and in chained of all time but Atlas was release from his chains with the help of the goddess Persephone Kratos was defeated by Charon, and subsequently thrown into Tartarus. As he attempted to make his way out of the depths, Kratos fought his way through hordes of enemies, came across the prison of the Tartarus, and obtained the Gauntlet of Zeus, which aided his escape and defeated Persephone and chained up Atlas and made him hold the world on his shoulder after he wreck the pillar of the world.
After his fight with Zeus and the death of Athena, Kratos went back to the Temple of the Fates and used the Mirror of Destiny to travel back in time, to where the first Great War erupted. Upon convincing Gaia and the Titans to return to the present, where Zeus was weakened, Kratos brought forth the Second Titanomachy as the Titans climbed Mount Olympus in order to destroy the Gods.
In order to secure the aid of Hephaestus, Kratos traveled to the Pit of Tartarus to find the Omphalos Stone. Hephaestus sought to use the stone to forge a Special Weapon for Kratos to use in his battle against Zeus. During his journey, Kratos encountered Cronos, who was angered by Kratos' involvement in Gaia's death. As the two engaged in battle, Kratos eventually proved superior, as he slew the Titan with the Blade of Olympus by thrusting it into his forehead. He then escaped the hellish area with the Omphalos Stone and returned to The Forge.